Converter connections

ABSTRACT

A single or multiphase converter connection includes a controlled first converter having an AC side and a DC side. A first reactor connects the DC side to a DC source. A capacitor is connected to the AC side of the first converter. A controlled current circuit is formed of a second reactor and controlled rectifiers connected to the second reactor to control the current therethrough. A control means is provided for firing said rectifiers with an adjustable delay angle so that the current circuit generates a variable capacitive current.

United States Patent Hermansson et a1.

CONVERTER CONNECTIONS Inventors: Bo Hermansson; Kjeld Thorborg, both ofVasteras, Sweden Assignee: Allmanna Svenska IElektriska Aktiebolaget,Vasteras, Sweden Filed: May 7, 1971 Appl. No.: 141,173

Foreign Application Priority Data May 28, 1970 Sweden ..7335/70 US. Cl..321/27 R, 321/27 MS, 321/45 R, 323/101, 323/123 Int. Cl. ..H02m G05fField of Search ..321/5, 27 R, 27 MS, 45 R; 219/l0.77, 10.79; 323/101,109, 123, 127

References Cited UNITED STATES PATENTS 6/1967 Germann et al. ..321/45 R2/1971 Wood ..321/45 R [451 Oct. 17, 1972 3,551,799 12/1970 Koppelmann..323/127 X 3,558,915 111971 Wood et a1 ..321/45 R 2,341,280 2/1944Ludbrook ..323/109 FOREIGN PATENTS OR APPLICATIONS 996,587 6/1965 GreatBritain. ..321/45 R Primary Examiner-William II. Beha, Jr.Attorney-Jennings Bailey, Jr.

[57] ABSTRACT A single or multiphase converter connection includes acontrolled first converter having an AC side and a DC side. A firstreactor connects the DC side to a DC source. A capacitor is connected tothe AC side of the first converter. A controlled current circuit isformed of a second reactor and controlled rectifiers connected to thesecond reactor to control the current therethrough. A control means isprovided for firing said rectifiers with an adjustable delay angle sothat the current circuit generates a variable capacitive current.

7 Claims, 6 Drawing Figures PATENTEDum 1? m2 SHEEI 2 BF 3 IN VENT 0R.

PATENTED 081 1 71912 SHEET 3 [IF 3 llllll INVENTOR. g mHJES D BY MJEZLZCONVERTER CONNECTIONS Prior art converter connections are well known andsome thereof are especially useful for inductive loads, for example forfeeding induction furnaces. The load in this case is connected inparallel with a capacitor bank dimensioned so that the paralleloscillating circuit acquires the desired frequency. The converteroperates as an inverter and feeds alternating voltage energy to theload. The frequency of the load voltage will therefore be substantiallyequal to that of the parallel oscillating circuit. The oscillatingcircuit operates as a commutating alternating voltage source. By makingthe control angle of the converter rectifiers variable in relation tothe load voltage, the amplitude of the latter voltage can be madecontrollable.

However, if the frequency should be variable, this can only be effectedby making the reactances of the oscillating circuit elements variable,for example by connecting different numbers of capacitors in thecapacitor bank. Thus for practical and economical reasons which caneasily be understood, frequency control is only possible in few andlarge steps.

SUMMARY OF THE INVENTION However, with a converter connection accordingto the invention stepless control of the frequency is easily obtainedover a large range as well as the possibility of accurately guiding orregulating the frequency to the desired value. At the same time voltagecontrol is retained. A converter connection according to the inventionincludes a controlled first converter having an AC side and a DC side, afirst reactor connecting its DC side to a DC source, a capacitorconnected to the AC side of the first converter, a controlled currentcircuit formed of a second reactor and controlled rectifiers connectedto the second reactor to control the current therethrough and a controlmeans for firing said rectifiers with an adjustable delay angle so thatthe current circuit generates a variable capacitive current.

BRIEF DESCRIPTION OF THE DRAWINGS DESCRIPTION OF THE PREFERREDEMBODIMENTS In FIG. 1 the converter S1 is fed over the smoothing reactorR1 (which is large enough for the direct current to be continuous) froma direct voltage source 1 consisting, for example of an accumulatorbattery or a diode or thyristor rectifier fed from an alternatingvoltage network. The converter consists of the four thyristors 11-14which are arranged to be conventionally fired in pairs (11, 12 and 13,14) and alternately by a control pulse device not shown in the drawing.The firing is arranged to take place with a delay in rela tion to thevoltage on the alternating voltage side of the converter of between 90and 180. The converter therefore operates as an inverter and suppliesenergy from its direct to its alternating voltage side. To the latter aparallel circuit is connected comprising a capacitor 2 and a reactor 3as well as a resistor 4 which indicates the equivalent resistance of theload object fed by the converter. The load object may be inductive, inwhich case it is connected in parallel with the capacitor 2 as shown. Ifthe load object is purely resistive the inductance of the reactor R2 canbe used for the oscillating circuit and no special reactor 3 need thenbe used. The converter may of course also be used to supply analternating voltage network, for example in an emergency current unit.

A controlled current circuit having connection terminals 5, 6 is alsoconnected to the alternating voltage output of the converter. Thisconsists of a second converter S2 with thyristors 21-24 and the reactorR2, connected to the direct voltage side of the converter S2. Thethyristors are controlled by a control pulse device, not shown, inconventional manner so that a variable direct voltage is obtained overthe reactor. If

the circuit (reactor thyristors) were to be loss free, the ignitiondelay of the thyristors would always be exactly which would give thedirect voltage the value zero and a finite current through the reactor.However, the circuit always has certain losses. The ignition delay musttherefore be slightly less than 90 for a current to fiow and by varyingthe ignition delay the magnitude of the reactor current can be adjusted.The current taken out from the AC side is thus nearly 90 phase-displacedafter the alternating voltage and is thus almost a purely inductivecurrent. The controlled current circuit S2, R2 thus functions as aconsumer of a variable inductive current or, which is the same thing, itgenerates a variable capacitive current.

By decreasing the ignition delay in S2 from 90 the current in R2 isincreased and the inductive load current to the circuit thus alsoincreases. In order to maintain the balance between inductive andcapacitive current in the circuit, the capacitive current must alsoincrease and this is done because the frequency of the load voltageincreases. This frequency can thus easily be controlled substantiallywithout losses within wide ranges by controlling the ignition delay ofthe converter S2.

FIG. 2 shows another embodiment of the controlled current circuit. Thereactor R2 is here connected in series with the anti-parallel connectedthyristors 25, 26 and an (inductive) alternating current flows throughit, the magnitude of which can be varied by regulating the thyristors.

FIG. 3 shows an embodiment inwhich the reactor R2 is provided with acentral tap 29 connected to one connection terminal 6 of the circuit.The free ends of the reactor are connected through the oppositelydirected thyristors 27, 28 to the second connection terminal 5 of thecircuit. The function of the circuit is principally the same as that ofthe circuit shown in FIG. 1.

FIG. 4 shows the load voltage (U1), the alternating current (II) fromthe converter S1, the voltage (U over the reactor R2, the alternatingcurrent (I2) of the converter S2 and the reactor current (1,) in thecircuit according to FIG. 1. The reference directions are clear fromFIG. 1. As can be seen from I1, the ignition of the thyristors in S1takes place at an angle [3 before each passage of the alternatingvoltage U1 through zero. B must be so much greater than zero that thecommutated thyristors have time to recover before once again receivingreverse blocking voltage. By altering B the amplitude of the alternatingvoltage can be regulated. Since the feeding direct voltage from thesource 1 is constant the alternating voltage U1 will be proportional tol/cos B, from which it follows that B must be less than 90 so thattheoretically infinite alternating voltage amplitude will not beobtained. As is clear from U the thyristors in S2 are ignited at anangle a before each passage through zero, where a is somewhat greaterthan 90. The magnitude of the reactor current 1,, and thus, as shownabove, the frequency of the alternating voltage are regulated byaltering a.

FIG. 5 shows the same magnitudes in the embodiment according to FIG. 2.The reactor R2 has an alternating current flowing through it, so that I2I The angle a can be regulated in this case between 0 and 90, thuschanging I between zero and its maximum value and consequently alteringthe frequency from its minimum to its maximum value,

FIG. 6 shows a control pulse device to effect the desired control of thecircuit in FIG. 1. Of course, the most obvious thing would be to sensethe zero-passages of the alternating voltage U1 and in relation to thesezero passages to emit ignition pulses to the thyristors at the momentsdefined by the angles a and [3. Such a method is relatively complicatedand presents certain problems because of the variable frequency. In thecontrol circuit shown in FIG. 6 the need for locking the firing times tothe alternating voltage has been eliminated, thus eliminating theseproblems and considerably simplifying the control pulse device.

A free-running pulse generator 30 delivers pulses I having a variablefrequency which is four times the desired alternating voltage frequency.The pulses trip a switch 31 alternately to one position and zeroposition. Thus alternate pulses from the generator, through thedifferentiating circuit 32, switch the switch 33 so that two thyristorsin the converter 32 are ignited over the amplifiers 34 and 35,respectively. The thyristors 23, 24 and the thyristors 21, 22 areignited alternately. Exactly between each such ignition a pulse isobtained from the zero output of the switch 31 which through thedifferentiating circuit 36 momentarily closes the electronic contact 37.The capacitor 38 is thus instantaneously discharged, after which untilthe next discharge it is charged with constant current from the source40 over the resistor 39. The capacitor voltage, which is thus asaw-tooth voltage, is compared in the comparison device 41 with avariable reference voltage which is obtained from the resistor 43connected to the source 42. When the capacitor voltage exceeds thereference voltage a pulse is obtained from the level discriminator 44,whereupon one of the two thyristor pairs 11, 12 or 13, 14 in theconverter S1 is ignited by the The desired frequency of the alternatingvoltage is thus set by adjusting the frequency of the generator 30. Itis then found that the angle a, i.e. the phase displacement between theignition pulses to S2 and the altemating voltage, is automaticallyadjusted to such a value that the circuit S2, R2 generates exactly thecapacitive current required for the frequency in question. The phasedifference (a B) between the control pulses to S2 and the control pulsesto S1 may be altered by altering the reference voltage from the circuit42, 43 and thus, since with constant frequency a is constant, B can bevaried and the desired alternating voltage amplitude set.

The control device shown may of course be supplemented by closedregulating systems for voltage or frequency or both and of coursealternative or similar known control devices according to the aboveprinciples may be used.

The converter connection according to the invention is described andshown in a single phase embodiment. However, it can also be applied tomultiphase converters. In the alternative according to FIG. 1 theconverters S1 and S2 are in that case replaced by, for examplethree-phase converters. In the alternative according to FIGS. 2 and 3circuits in accordance with these figures in, for example, three-phasearrangement, are connected between each pair of phase conductors of S1(triangle connection) or between each phase conductor and an artificialzero point (star connection).

Iclaim:

l. A self-commutated inverter comprising a controlled first converterconnection, comprising controlled rectifiers, a DC source, saidconverter connection having an AC side, a first reactor connecting theDC side of the first converter connection to the DC source, a capacitorconnected to the AC side, a variable inductance connection connected tosaid AC side and comprising a second reactor and controlled rectifiersconnected to the second reactor to control the current therethrough, acontrol pulse means connected to said first converter connection and tosaid variable inductance connection and comprising a freerunningoscillator which is arranged to supply a first pulse train to said firstconverter connection, to ignite the controlled rectifiers thereof, saidpulse train having a frequency corresponding to the frequency of theoscillator and to the desired inverter output frequency, said controlpulse means being arranged to supply a second pulse train to thevariable inductance connection for ignition of the controlled rectifiersthereof, said second pulse train having a frequency equal to that ofsaid first pulse train, corresponding pulses of said first and secondpulse trains being phase displaced in relation to each other.

2. A self-commutated inverter according to claim I, said variableinductance connection comprising an anti-parallel connection of twocontrolled rectifiers connected in series with the second reactor.

3. A self-commutated inverter according to claim 1, in which the secondreactor is provided with a central tap which is joined to a firstconnection terminal of said variable inductance connection, and twooppositely directed controlled rectifiers connecting both ends of thereactor to a second connection terminal of the variable inductanceconnection.

nected between each phase output and an artificial zero point.

7. A self-commutated inverter according to claim 1, in which thevariable inductance connection comprises a second controlled converterconnection, the AC side of which is connected to the AC side of thefirst converter connection, said second reactor being connected betweenthe DC terminals of said second controlled converter connection.

1. A self-commutated inverter comprising a controlled first converterconnection, comprising controlled rectifiers, a DC source, saidconverter connection having an AC side, a first reactor connecting theDC side of the first converter connection to the DC source, a capacitorconnected to the AC side, a variable inductance connection connected tosaid AC side and comprising a second reactor and controlled rectifiersconnected to the second reactor to control the current therethrough, acontrol pulse means connected to said first converter connection and tosaid variable inductance connection and comprising a freerunningoscillator which is arranged to supply a first pulse train to said firstconverter connection, to ignite the controlled rectifiers thereof, saidpulse train having a frequency corresponding to the frequency of theoscillator and to the desired inverter output frequency, said controlpulse means being arranged to supply a second pulse train to thevariable inductance connection for ignition of the controlled rectifiersthereof, said second pulse train having a frequency equal to that ofsaid first pulse train, corresponding pulses of said first and secondpulse trains being phase displaced in relation to each other.
 2. Aself-commutated inverter according to claim 1, said variable inductanceconnection comprising an anti-parallel connection of two controlledrectifiers connected in series with the second reactor.
 3. Aself-commutated inverter according to claim 1, in which the secondreactor is provided with a central tap which is joined to a firstconnection terminal of said variable inductance connection, and twooppositely directed controlled rectifiers connecting both ends of thereactor to a second connection terminal of the variable inductanceconnection.
 4. A self-commutated inverter according to claim 1, in whichthe converter connection is n-phase and its AC side is provided withn-phase outputs, and having n identical variable inductance connectionsconnected to the phase outputs.
 5. A self-commutated inverter accordingto claim 4, in which the variable inductance connections are connectedbetween each pair of phase outputs.
 6. A self-commutated inverteraccording to claim 4, in which the variable inductance connections areconnected between each phase output and an artificial zero point.
 7. Aself-commutated inverter according to claim 1, in which the variableinductance connection comprises a second controlled converterconnection, the AC side of which is connected to the AC side of thefirst converter connection, said second reactor being connected betweenthe DC terminals of said second controlled converter connection.